There are two major types of compression drivers, the first utilizing a dome diaphragm, and the other using an annular flexural diaphragm. The majority of modern annular diaphragms are made of polymer films. The advantage of annular diaphragms is the smaller radial dimensions of the moving part of the diaphragm compared to the dome diaphragms having the same diameter of the moving voice coil. The small radial clamping dimension of the annular diaphragm shifts the mechanical breakup resonances of the diaphragm to higher frequencies where they can be better mechanically damped, since the damping is more efficient at high frequencies in polymer films. Better damping is indicative of the smoother frequency response and lower nonlinear distortion generated by diaphragms' breakups at high frequency.
In a compression driver, the diaphragm is loaded by a compression chamber, which is a thin layer of air separating the diaphragm from a phasing plug. The phasing plug receives an acoustical signal produced by the vibrating diaphragm and directs it to the exit of the compression driver. One of the primary features of a conventional compression driver is the difference between the larger effective area of the diaphragm and the smaller area of the compression chamber exit. The smaller area of the compression chamber exit increases its input impedance that loads the diaphragm. In theory, a compression driver reaches maximum efficiency when the mechanical output impedance of the vibrating diaphragm equals the loading impedance of the acoustical load. This assumption is approximate because, in reality, both impedances are different, complex, frequency-dependent functions.
A typical compression chamber has a single or multiple narrow exits expanding to the exit of the compression driver. Two types of linear distortion may occur in the compression chamber. One type is the attenuation of the high frequency sound pressure signal caused by the compliance of air trapped in the compression chamber. The volume of entrapped air is characterized by an acoustical compliance which is proportional to the volume of compression chamber. Acoustical compliance acts as a low-pass filter of the first order and it mitigates the high frequency signal. The second type of distortion is the irregularity of the high frequency sound pressure level (SPL) frequency response caused by air resonances in the compression chamber. The latter typically interact with high frequency mechanical resonances of the vibrating diaphragm.